Dipole response in ^128,130Te below the neutron threshold

Background: Numerous studies of the ground-state decay of the pygmy dipole resonance (PDR) have been carried out in the past. However, data on the decay of the PDR to low-lying excited states is still very scarce due to limitations of the sensitivity to weak branching transitions of experimental setups. 

Purpose: We present a detailed examination of the low-energy dipole response of ¹²⁸Te and ¹³⁰Te below their neutron separation thresholds of 8.8 and 8.5 MeV, respectively. 

Methods: Photonuclear reactions with the subsequent γ-ray spectroscopy of the decay channel with continuous-energy bremsstrahlung at varying endpoint energies and linearly polarized quasimonochromatic γ-ray beams with energies ranging from 2.7 to 8.9 MeV in steps of roughly 250 keV were used for probing the decay behavior of the low-energy dipole response in ¹²⁸Te and ¹³⁰Te. In addition, (γ^→,γ′γ′′) reactions were used to study the population of low-lying states of ¹²⁸Te. 

Results: Spin-parity quantum numbers and reduced transition probabilities are determined for individual photo-excited states. The analysis of average decay properties for nuclear levels in narrow excitation-energy bins enable the extraction of photoabsorption cross sections, average branching ratios to the 2⁺₁ state, and the distinction between E1 and M1 transitions to the ground state and to the 2⁺₁ state accounting for resolved and unresolved transitions. 

Conclusions: Above 5 MeV, the experimental data are in reasonable agreement to calculations within the quasiparticle phonon model. The major fraction of the ground-state decay channel is due to E1 transitions, while less than 5-10% stem from M1 transitions. Furthermore, first direct experimental evidence is provided that the population of the 2⁺₁ state of ¹²⁸Te via primary γ-ray transitions from excited states in the PDR region from 5 to 9 MeV is dominated by E1 transitions of 1^- states.